This invention relates to a circuit operative at a first or second time constant. Particularly, it relates to such circuits which utilize RC circuits fed by a pulse generator G. Such circuits are common in dynamic digital/analog converters.
In such dynamic digital/analog converters, the analog value is generally derived by integration from a pulse sequence. For this purpose an integrating circuit having a time constant p is generally utilized. These integrating circuits also go under the name of lowpass filter. This type of circuit, namely a series circuit of a resistor and a capacitor, takes the form of an unsymmetrical four-pole integrator whose input is alternately connected to a first and second reference potential by means of the pulse generator. The output voltage U.sub.A of the four-pole network, which constitutes the output analog signal, is of course derived from the capacitor. A long time constant, as is well known, causes little ripple to appear on the output signal which is extremely desirable. However it has the disadvantage that the circuit introduces delay. Therefore if a rapid change between different analog values and a low ripple are to be realized, the time constant must be changed. The analog value is changed at a low time constant and the circuit is then switched to a higher time constant. Two basic possibilities have been realized heretofore for switching the time constant of RC circuits (which RC circuits are to include differentiating circuits as well as the above-described integrator circuit): switching of the value of the capacitor C or that of the resistor R. Either parallel or series circuits may be used to implement the switching.
The disadvantages of the above two methods of switching the time constant will be explained with reference to an integrating circuit even though the same considerations are valid for a differentiating circuit.
First prior-art circuit is shown in FIG. 1. It constitutes a series resistor R, a first capacitor C across which the output voltage U.sub.A is to be furnished, and a capacitor C' which may be connected in parallel with a capacitor C by closing of a switch. However, if an additional capacitor having an arbitrary initial charge is abruptly connected in parallel with capacitor C, the transient arising due to the exchange of charge between the two capacitors upon closing of the switch causes a large error to be introduced into the output voltage. This condition cannot be tolerated in most applications.
In accordance with the method of FIG. 2, an additional resistance R' is alternately connected in series with the main resistor R or is short-circuited. Here the difficulty exists that both sides of switch S are at varying potentials.